Voice communication devices such as cell phones, wireless phones and devices other than cell phones have become ubiquitous; they show up in almost every environment. These systems and devices and their associated communication methods are referred to by a variety of names, such as but not limited to, cellular telephones, cell phones, mobile phones, wireless telephones in the home and the office, and devices such as Personal Data Assistants (PDAs) that include a wireless or cellular telephone communication capability. They are used at home, office, inside a car, a train, at the airport, beach, restaurants and bars, on the street, and almost any other venue. As might be expected, these diverse environments have relatively higher and lower levels of background, ambient, or environmental noise.
The term “wind noise” is used to describe several different ways that wind can be generated. For example, wind can cause a loose shutter to bang against a house or it can cause a flag to rustle and snap. In these cases, the wind has caused an object to move, and the motion makes a sound. In other cases, wind moving past an object can create a howling sound, even though the object does not vibrate. Here, the sound is caused by turbulence that is created in the moving air as it passes by the object. This turbulence, which cannot be seen, is very similar to the turbulence in a fast-moving stream as the water flows around and over large rocks. We have all experienced this kind of wind noise while inside a house during a windstorm. The sound of the howling wind originates in the turbulence of air motion past the walls and roof.
The form of wind noise that most interferes with our ability to hear and communicate is the noise generated by air flow around our own head. Here the sound is generated within centimeters of our ears, and may be heard at quite a high level because of this close proximity [1]
Wind noise has been studied extensively and many solutions have been proposed for hearing aids, Bluetooth headsets, car kits, cell phones etc.
Wind noise exhibits some properties and features that are not common to other types of noise encountered in our daily lives. Depending on the wind speed, direction, physical obstructions like hats, caps, hand etc the characteristics of wind noise vary greatly. For these reasons, it is difficult to detect and classify the presence of wind noise from other environmental noises.
It is known art to reduce wind noise by mechanical means such as foam, scrims etc. To be sufficiently effective, the mechanical means must be thick which might make the device look bulky. Also these solutions add up to the Bill of Materials (BoM) of the device. This can be undesirable.
However, certain factors make wind noise unique. Wind noise predominantly is a low-frequency phenomenon. Many of the known art technologies detect wind noise using the property of low correlation of the wind noise between multiple microphones separated spatially.
Several attempts to detect wind noise are known in the related art. US patent US2002/037088, assigned to Dickel et al, detects wind noise by computing the correlation between signals received at the two microphones. Turbulence created at the two microphones, without any obstructions, causes signals with low correlation. However, our studies showed that obstructions in the vicinity of the microphone result the correlation to be high.
European patent EP 1 339 256 A2, assigned to Roeck et al, uses several of the well know wind noise properties like high energy content at low frequencies, low auto-correlation at two microphones and high signal amplitudes. However, this approach also suffers from the same drawbacks discussed above.
European patent application EP 1 732 352 A1, assigned to Hetherington et al, uses multiple microphones where power levels in different microphones are compared. When the power level of the sound received at the second microphone is less than the power level of the sound received at the first microphone by a predefined value, wind noise may be present. However, this approach requires one of the microphones to be directional with high directivity index and the other microphone to be Omni-directional with low directivity index.
Hence there is a need in the art for a method of wind noise detection and classification that is robust, suitable for mobile use, and inexpensive to manufacture.
It is an objective of the present invention to provide methods and devices that overcome disadvantages of prior art wind noise detection and classification schemes.